The present invention relates to pressure-relief valve devices. Various hydraulically-operated units of mine equipment need to be protected against excessive pressure. This is especially the case with hydraulic pit props which support the roof of a mine working. Normally, such props would utilize an oil-water emulsion as pressure fluid at a pressure not in excess of 600 bars and conventionally in the range 200-600 bars. In adverse conditions, the pressure in the working chamber of a hydraulic prop may rise suddenly and normally a small compact pressure relief valve device is connected to the working chamber to relieve the pressure and protect the prop should the pressure rise beyond a certain critical valve. The known valve devices operate entirely automatically and close and open in accordance with the prevailing pressure. One example of a pressure relief valve device is described in the specification for German Gebrauchsmuster No. 1945206. Exacting demands are made on the valve devices since they are expected to perform reliably over long periods of time and they are expected to remain highly sensitive to pressure variation. Such valve devices are moreover expected to perform efficiently in the harsh conditions encountered in mine workings and thus need to be especially durable.
A general object of the present invention is to provide an improved valve device and, above-all, to provide a device in which the reliability and durability are improved.